Keeping time Fruit and vegetables are still alive after they're picked, continuing to respond to changes in light across the day and night, a study has found.

The findings suggest fruit and vegies should be stored using lighting to simulate day and night to keep them fresh and nutritious, US scientists report in the latest edition of the journal Current Biology.

Fruits and vegetables respond to light signals and change their biology in ways that may affect health value and insect resistance, says Professor Janet Braam of Rice University in Houston, Texas.

"Perhaps we should be storing our vegetables and fruits under light-dark cycles and timing when to cook and eat them to enhance their health value," Braam says.

She says the research builds on previous work that identified how plants use their 'circadian clock' to repel insect pests.

Last year, Braam and colleagues found that mustard plants use changing light levels to control physiological processes, including the production of glucosinolate -- a natural anti-herbivore compound.

"The chemicals accumulate throughout the day when the insects are about to eat," says Braam.

In this latest research, Braam and colleagues turned their attention to the humble cabbage.

"We realised that the mustard plant is related to some of our crop plants such as cabbage, broccoli and cauliflower, and so we considered the possibility that some of these chemicals might also be cycling in these vegetables."

Scientists have known that plants can metabolise and survive after they have been removed from the parent plant. But whether they continue to maintain a circadian rhythm was unknown.

"We thought we'd take a chance," says Braam. "We have to admit we were a bit surprised that this worked as well as it did."

Simulating day and night

The researchers placed cabbage discs in either simulated 12-hour day, 12-hour night cycles (known as entrainment), or in constant light or dark storage. Over several days, the cabbage discs were analysed and exposed to a known predator -- cabbage looper larvae (Trichoplusia ni).

The researchers found that the entrained cabbages, maintained their circadian rhythms and glucosinolate levels for up to a week. They also noted that the weight gain of the larvae fed the light-dark cabbage leaves was half that of those fed the leaves stored in constant light or dark.

Braam says the results show cabbage can maintain its circadian clocks to continue to produce compounds such as glucosinolate.

The researchers repeated the experiment, storing the cabbages at 4°C, and found the results to be similar.

"The levels of the chemicals were reduced, but they still cycled as when they were stored at 22°C," says Braam.

The researchers then turned their attention to lettuce, spinach, zucchini, sweet potato, carrots and blueberries to test how broad the effect is.

"The results have shown that yes we can re-entrain their circadian clock," says Braam.

Benefits to humans

According to Braam, the compounds used by cabbages and other fruit and vegetables to prevent pest attack are also beneficial to humans.

"These same chemicals that are used in pest defence are also important metabolites for people. These are chemicals that are among the most potent natural anti-cancer compounds known," she says.

Braam says the results of the study could eventually lead to the use of light cycles when storing fruit and vegetables.

Does this mean tomorrow's refrigerators could use lighting to simulate day-night cycles to maintain the nutritional value of fresh fruit and vegetables?

"Possibly," says Braam. "We haven't yet worked out the optimal levels for light-dark cycles and temperature."

"We've only just scratched the surface of this. But it might not take much light to keep this cycle going."

"Plants can respond to really low levels of light, but these are some of these things that we still have to work out."